1.A.136.  The Type 10 Protein Secretion System (T10SS) Family 

Gram-negative bacteria have evolved numerous pathways to secrete proteins across their complex cell envelopes. Palmer et al. 2021 described a protein secretion system that uses a holin membrane protein in tandem with a cell wall-editing enzyme to mediate the secretion of substrate proteins from the periplasm (or cytoplasm?) to the cell exterior. The identity of the cell wall-editing enzymes involved was found to vary across biological systems. For instance, the chitinase secretion pathway of Serratia marcescens uses an endopeptidase to facilitate secretion, whereas the secretion of Typhoid toxin in Salmonella enterica serovar Typhi relies on a muramidase. Various families of holins are also predicted to be involved. Genomic analysis indicates that this pathway is conserved and implicated in the secretion of hydrolytic enzymes and toxins for a range of bacteria. The pairing of holins from different families with various types of peptidoglycan hydrolases suggests that this secretion pathway evolved multiple times. Palmer et al. 2021 suggested that the complementary bodies of evidence presented are sufficient to propose that the pathway be named the Type 10 Secretion System (TXSS). Chitinase A, a tightly regulated virulence factor of Salmonella enterica serovar Typhimurium, is actively secreted by a Type 10 Secretion System (Krone et al. 2023).